To prevent unauthorized duplication or alteration of documents, frequently there are special indicia or a background pattern provided for sheet materials such as tickets, checks, currency, and the like. The indicia or background pattern is imposed upon the sheet material usually by some type of printing process such as offset printing, lithography, letterpress or other like mechanical systems, by a variety of photographic methods, by xeroprinting, and a host of other methods. The pattern or indicia may be produced with ordinary inks, from special inks which may be magnetic, fluorescent, or the like, from powders which may be baked on, from light sensitive materials such as silver salts or azo dyes, and the like. Most of these patterns placed on sheet materials depend upon complexity and resolution to avoid ready duplication. Consequently, they add an increment of cost to the sheet material without being fully effective in many instances in providing the desired protection from unauthorized duplication or alteration.
Various methods of counterfeit-deterrent strategies have been suggested including Moire-inducing line structures, variable-sized dot patterns, secondary images, see-throughs, bar-codes, diffraction based holograms and more. However, none of these methods employs a reliable, readable secondary image in a primary image without the former having influence on the quality of the latter, and the provision of additional security benefits derived therefrom.
Conventional systems for coding and decoding indicia on printed matter produce a parallax panoramagram image or a scrambled image. Such a conventional system is described in U.S. Pat. No. 3,937,565, to A. Alasia issued Feb. 10, 1976, now expired. The indicia were produced photographically using a lenticular line screen (i.e. a lenticular screen) with a known spatial lens density (e.g. 69 lines per inch).
Photographic, or analog, production of coded indicia images has the drawback of requiring a specialized camera. Also, the analog images are limited in their versatility in that an area of counterfeit deterrent indicia an generally noticeable when surrounded by foreground (secondary) images. Also, it is difficult to combine several secondary images, with potentially different parameters, due to the inability to effectively re-expose film segments in generating the counterfeit determent, photographic image.
Various reproduction technologies, such as printed or unprinted (electronic) technology, used for distribution of visual information, are based on screening of the image. In these technologies, the picture is divided into a set of systematically coordinated elementary dots, pixels, etc., the size of which are below the resolution of the human eye. Referring to FIGS. 1A-1F, examples of various printing screens of the prior art are shown which may be used to produce image 100 having different shades. In FIG. 1A, a portion 102 of image 100 is enlarged to show the effect of the different screening techniques as shown in FIGS. 1B-1F. These screens make reproduction possible, but at the same time decrease reproduction quality of the image when compared to the original image, rendering the reproduced image "noisy".
Furthermore, imperfections of different systems and media, used for reproduction, such as ink, print media (e.g., paper, plastic, etc.), electron-beams, display pixels, etc., neither allow the creation nor the grouping of the elementary information-holders, such as dots, pixels, etc., in full accordance with the clear theoretical requirements, but only with fewer or greater distortion. This further increases the "noise" in the resultant image.
In the case of four color reproduction, either electronic or printed, there is also a decrease in the quality of the picture due to the millions of color-shades of the original image that must be reproduced by using three colors only, represented by optically imperfect inks.
As shown in FIGS. 2A and 2B, the above factors, and a variety of other factors, produce the result that none of the elementary dots 202-210 have theoretically perfect geometrical form, position and size after printing 202A-210A.
Screening and coloristic questions are crucial points of multicolor reproduction technology. To solve the coloristic problems, two international standards have been established. These are the Red-Green-Blue (RGB) and Cyan-Magenta-Yellow-Black (CMYK) standards which are used universally. Six color reproduction is also used in limited applications.
Using a conventional 80 line/cm printing screen, four different ink-dots may be printed in an area of 0.125 mm.times.0.125 mm (0.005 in..times.0.005 in.) in exact size, geometric form, position and thickness. This increase in resolution exacerbates the problem, because decreasing the size of the elementary dots or pixels (i.e. increasing the resolution of the screen) decreases the "noise" of the image, but undesirable influences of the imperfections of the applied materials and processes increases. The closer the resolution of the screen to the resolution of the reproduction process (i.e. to the limits of printability), the more technological imperfections undesirably impact the produced image.
In order to reduce the undesirable consequences of these imperfections, they must be taken into consideration in advance during the reproduction process.
For this reason, the original image may be digitized or scanned, and divided into elementary pixels in a continuous tonal mode, by using an appropriate screen. The size of all pixels are the same, although the density of pixels may be different, according to the actual image.
Once the theoretical density has been modified accordingly, the pixels may be converted from a continuous mode into a bit-map mode. In the bit map mode, the size of the dots are different but the overall density of the dots are equal. This is preferred because, during the printing (with the exception of the gravure printing), the thickness or printable density of the printable ink fill is the same overall. As a result, a continuous tone pixel having the maximum area of 0.125.times.0.125 mm, (0.005 in.times.0.005 in, using the 80 line/cm screen) and a density of 25%, for example, is replaced with a screen dot having an optical equivalence, covering only 25% of the same area, but having an equivalent maximum density.
Some conventional reproduction processes and devices use continuous tone pixels, such as etched rotogravure, electronic display, and some digital printers. Other reproduction processes use screen-dots, like offset printing, and most digital printing processes. Further processes use a combination of both continuous tone and screen dots, for example, such as, intaglio printing, and gravured rotogravure printing.
The process of conversion of continuous tone mode into a bitmap mode is a complex procedure and has a primary importance in screening technology. This is because the theoretical density of the continuous tone elementary pixels, received after scanning, is modified in advance according to the technological imperfections of the further reproduction processes.
For example, in offset print reproduction the technological imperfections may include:
1. Distortions in form and size of the converted dots through the further reproduction processes, such as: PA1 2. Optical imperfections of the applied inks. PA1 inhomogeneities of the paper surface, the rubber blanket and the printing ink, PA1 distortions resulting from the impression power in the print zone, PA1 the mechanical inaccuracies in the printing apparatus, and PA1 deformations in the printing paper.
converting the continuous tone pixels into screen dots PA2 creating of dots in the image sets in which moire effects may occur PA2 film exposure and processing, PA2 copying on a printing plate, PA2 processing of the printing plate, and PA2 the printing process.
Most of the distortions of elementary screen-dots occur in the printing process. As a result, unpredictable effects may occur, such as:
Different printing technologies have different imperfections, characteristic to each particular printing process. Therefore, to compensate for these different imperfections, different screening technologies and screens have been developed.
For digital printing the screening has an even greater importance. There are different versions of digital printing technologies, such as the laser, inkjet, dye sublimation, magnetographic, electrostatic, etc. Thus, as these processes are still emerging, they have significantly more imperfections than traditional printing processes.
The correction of technological imperfections is even more complicated in security printing. The smaller or thinner the printed element is, the greater the relative distortion in the printing process, and the more difficult compensation of these distortions.